[5.x] Matrix field entry type groups

[turnstylerj]

Trying out the new Matrix as a Neo replacement on my current site, and one of things I'm missing is the ability to group blocks. It would be nice if we could group Matrix field entry types either via a UI like the "Customize Sources" in the main Entries panel sidebar or maybe with the same UI as Matrix currently has but we could either add an Entry Type or a Group and drag the group block where we'd like to separate blocks.